An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State: Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State: Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a pixel in a display.
Electrophoretic displays offer excellent viewing characteristics with extremely low power requirements. The intrinsic qualities of both reflective and transmissive electrophoretic displays are known to those skilled in the art and are responsible for continued interest in this technology. The object of the present invention is to provide an improved electrode configuration for a transmissive electrophoretic display.
A transmissive electrophoretic display is generally comprised of a regular lateral array of transmissive electrophoretic cells. Each cell is generally comprised of a suspension of charged pigment particles colloidally dispersed in a light-transmissive fluid that is contained between two parallel light-transmissive windows that are uniformly illuminated from the side opposite the viewer with a backlight. Each cell is also generally comprised of electrode elements disposed between the parallel windows that are arranged to transport and control the spatial distribution of the charged particles under the influence of an electric field. Each transmissive electrophoretic cell serves as a light valve since the light absorbing and/or light scattering characteristics of a cell depend on the spatial distribution of the particles within the cell.
Dalisa, et al., in U.S. Pat. No. 4,218,302, describe a transmissive electrophoretic display that uses particles to either allow or frustrate the total internal reflection of light at the interface between the interior of the rear window and the suspension fluid. In the regions of this interface where no particles are present, the conditions for total internal reflection are satisfied, and light from the source is reflected back towards the source and the viewer sees dark. In the regions of this interface where the particles are present, the conditions for total internal reflection are frustrated, and light from the source passes through the interface and the viewer sees light. The contrast offered by this approach therefore depends on the extent to which the particles can be forced to form an intimate, continuous layer at this interface.
In other disclosed embodiments of a transmissive electrophoretic display, visual contrast is achieved by either placing absorbing particles in, or removing absorbing particles from the light path between the backlight and the viewer. When a selected portion of the lateral area is fully covered by absorbing particles, light from the backlight is substantially absorbed and the viewer sees dark. When the lateral area occupied by the absorbing particles is reduced, light from the backlight can be transmitted and the viewer sees light. Consequently, the resulting performance of a transmissive electrophoretic display using this approach critically depends on the scheme used to alter the lateral area occupied by the absorbing particles.
Marshall et al., in U.S. Pat. No. 4,648,956, describes several embodiments of a transmissive electrophoretic display in which the lateral area occupied by the absorbing particles is controlled by using an array of horizontally disposed electrodes whose total area is small compared to the horizontal viewing area of the display. In these embodiments the suspension is contained between a transparent, large area upper electrode and a small area lower electrode. When the absorbing particles in a selected region of the display are collected on the small-area lower electrode, they only cover a small portion of the horizontal area of the selected region. As a result, most of the light from the source passes through the selected region without being absorbed and the viewer sees light. When the absorbing particles are drawn to the upper electrode, they substantially cover the horizontal area of the selected region so that most of the light from the source is absorbed before it can reach the viewer and the viewer sees dark.
Hou, in U.S. Pat. No. 5,298,833, describes a transmissive electrophoretic display in which the lateral area occupied by the absorbing particles is controlled by using a conductive mesh screen disposed between the backlight and the viewer. In this embodiment, the mesh screen covers the viewing area of the display and is immersed in a suspension comprised of black particles in a clear fluid. Particles are either drawn to, or removed from a selected area of the mesh by using patterned transparent electrodes disposed above and below the mesh. When the black particles are drawn to the mesh they sit on the mesh without completely covering its holes. Light from the source is transmitted through the holes and the viewer sees light. When the particles are removed from the mesh, they are drawn to cover the selected transparent electrode. Light from the source is substantially absorbed in the area covered by the electrode and the viewer sees dark.
In both of the last two cited references, the desired darkness of the dark state, along with optical density and packing fraction of the particles, fixes the required number of particles. In these embodiments, therefore, the brightness of the light state is limited by the horizontal area the required number of particles can be forced to occupy. This occupied area can be reduced by compacting the collected particles into a small volume, as was proposed in the previous embodiments, but this approach has serious associated difficulties.
To give the display a reproducible light and dark state, the particles must withstand repeated switching between a collected state and a dispersed state without significant agglomeration or sticking. A small collection volume necessitates a highly compacted collected state and therefore places extreme demands on the long-term stability of the particles. Using a small area collection electrode also increases the voltage required to place the amount of charge on this electrode that is necessary to collect many particles.
There is a continuing need in the art for a low-power transmissive electrophoretic display with high contrast and brightness. It is therefore an object of the present invention to provide transmissive electrophoretic display having improved contrast and brightness with reduced switching voltage and power requirements. Other objects and advantages will become apparent from the following disclosure.